The present invention concerns structural cables used in civil engineering works. It is useful in particular in the fields of pre-stressing cables, stay cables or suspension bridges.
Modern structural cables are often made up of unitary tendons (wires or strands) arranged in substantially parallel bundles inside sheaths or exposed to the air. In view of the aggressivity of the external environment and the durability requirements, these cables generally have protective layers of anti-corrosion material: oil, galvanization, grease, wax, filling with elastomer materials or cement grout, or external metal or plastic sheathing.
These anti-corrosion protections are applied on structural cables (pre-stressing cables, bridge stay cables, suspension cables or any other structural cables) either to the unitary tendons or to sub-assemblies of tendons, or globally to the whole cable.
The protections applied to unitary tendons have several advantages in isolating the unitary tendons of the cables electrically and mechanically. This isolation prevents electrical bridging, generalized corrosion of one section of the cable by xe2x80x9cgangrene effectxe2x80x9d, and lateral contact between zones of curvature where pressure between tendons can produce stress concentrations detrimental to good static and dynamic behavior. It also prevents lateral contact in rectilinear sections when the tendons are free to move.
Individual protection of the tendons can take the form of sheathing: sheathed greased strands in the pre-stressing application, self-protected strands in stay cable structures, or coherent strands (EP-A-0 855 471).
In some applications the individually sheathed strands are positioned with their sheaths inside a mass of injected and hardened material, such as cement grout, which forms a mechanical spacer (see EP-A-0 220 113, EP-A-0 437 143, EP-A-0 465 303). When the strands are being put under tension the previously hardened mass maintains their transverse distribution in the sheath and prevents their deterioration in the curved sections of the cable.
An object of the present invention is to propose a method of protecting the tendons making up a structural cable which is compatible with diverse applications and diverse types of strand.
The invention thus proposes a structural cable for civil engineering works comprising at least one bundle of substantially parallel tendons contained in at least one plastic protective sheath section, wherein the plastic material of the sheath section extends between the tendons to form a coherent matrix for spacing the tendons.
The plastic protective sheath thus acts as a mechanical spacer for the strands and forms an individual sleeve for each strand.
A further advantage is to enable operations to inject material into the sheath after installation of the tendons to be dispensed with if appropriate. These operations are generally costly and difficult.
In particular embodiments of the structural cable according to the invention:
the sheath section is a piece of plastic material having substantially parallel longitudinal bores which preferably do not inter-communicate; it is possible that one or more of these longitudinal bores does not contain a stretched strand but is provided to fulfill other functions (such as conduits for measuring sensors or optical communications fibers, etc.);
the tendons are bare or individually protected metal strands;
the cable comprises a plurality of successive sheath sections assembled mechanically or by welding;
the sheath section has a circular or polygonal external cross section, for example, a shape allowing several bundles of tendons to be juxtaposed in a cable of relatively large cross section;
the plastic material of the sheath section comprises a combination of materials; such a combination can include a material providing surface strength towards the outside of the sheath section, and a visco-elastic material and/or a material providing a low coefficient of friction with the tendons towards the interior of the sheath section;
the sheath section, or an assembly of sheath sections assembled end to end, extends over substantially the entire length of a running section of the cable between, two end anchorages;
the cable forms a pre-stressing cable, a stay cable or a carrying cable for suspension bridges.
A further aspect of the present invention relates to a structural cable sheath section for civil engineering works which constitutes a semi-finished product before insertion of the tendons. This sheath section forms a coherent spacing matrix having substantially parallel longitudinal bores suitable for receiving a cable tendon in each bore.
The matrix may be made of plastic material. It can also include a section made of injected material such as cement grout. In the latter case it can comprise, for example, individual plastic tubes for receiving the tendons arranged inside an external tube, the injected material filling the external tube around the individual tubes.
Yet another aspect relates to a method of laying a structural cable in civil engineering works, wherein a sheath having at least one sheath section forming a coherent spacing matrix with substantially parallel longitudinal bores is used, tendons are respectively inserted in at least some of the bores of the sheath section, and the tendons are tensioned.
The tendons may be inserted in the bores of the sheath section by traction on guides previously passed through the bores, or by pushing. They may be tensioned individually or collectively. Each tendon can be extracted and/or replaced separately in case of need.